Object of http headers for AJAX calls.

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    Object, default = {}
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Whenever an AJAX call is made, e.g. in <a class="argument" href="w2form.save">.save()</a> or <a class="argument" href="w2form.request">.request()</a>, the httpHeaders object os passed along in the AJAX call.
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From the JQuery.ajax docs:
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An object of additional header key/value pairs to send along with requests using the XMLHttpRequest transport.
The header X-Requested-With: XMLHttpRequest is always added, but its default XMLHttpRequest value can be changed here.
Values in the headers setting can also be overwritten from within the beforeSend function.
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